1. Field of the Invention
The present invention relates to improvements of an imaging apparatus which forms high gloss images on a sheet by means of a chromogenic material like a color forming dye and thermoplastic resin.
2. Description of Prior Art
In recent years imaging apparatus for image forming by means of photosensitive and pressuresensitive sheets have been developed. Such apparatus use a medium sheet and a receiver sheet. The medium sheet is coated with microcapsules of 10 to 20 micrometers in diameter which are filled with a photohardenable material and a color forming dye (for example, leuco dye). The receiver sheet is coated with a developer material (acid, etc.) which causes the color forming dye to develop the color, and a thermoplastic resin. After exposing the medium sheet to light in order to form a latent image, the receiver sheet is placed on the latent image formed on the medium sheet and is pressurized. By pressurization, microcapsules which are not hardened are broken to cause the dye to spread over the receiver sheet, thereby forming the image. The dye develops the color by reacting with the developer material on the receiver sheet to form the image.
Then by heating the receiver sheet, color developing reaction is promoted and gloss is rendered to the image by the thermoplastic resin. (These methods of the prior art are proposed in the U.S. Pat. Nos. 4,554,235 and 4,576,891.)
When heating the receiver sheet which carries the color forming dye, heat conductivity varies depending on the type of the sheet, namely the kind of material and the thickness and, consequently, the total quantity of heat required for glossing also varies. The present applicant proposed an imaging apparatus which changes the heating temperature in accordance to the kind of sheet, in the Japanese Patent Application No. 63-3375.
However, high quality images cannot be obtained unless heating is carried out at significantly high temperatures, depending on the type of sheet. On the other hand, a high temperature sustained for a long period of time causes the dye, acid, thermoplastic resin, etc. on the sheet surface to adhere to other parts of the apparatus, leading to a defect called the offset which is the transference of the dye onto the receiver sheet. Further the high-temperature sublimation of the thermoplastic resin and other materials leads to the generation of gas. Prior art includes these problems.